1. Field of the Invention
The present invention is directed to water treatment. In particular the present invention is directed to the field of plasma spark discharge treatment of water and apparatus therefor.
2. Description of the Related Technology
Produced water refers to water that comes out of shale wells during the production of oil or gas, through a drilling process called hydraulic fracturing. During the drilling phase, the produced water is called flowback. Produced water can contain high concentrations of microorganisms, hydrocarbons, suspended solids, and dissolved inorganic solids.
Plasma spark discharge generators are used in the treatment of water. However, it is a challenge to produce a short-duration spark discharge in produced water because of the high electrical conductivity of produced water. The electrical conductivity of produced water is in the range of 100-200 mS/cm, which is approximately two to four times larger than that of sea water. As a result of this, the generation of a plasma spark discharge in the produced water is a significant challenge.
Previously, a number of researchers used gas bubbles in the generation of pulsed plasma spark discharge between two electrodes (i.e., HV electrode and ground electrode) in liquids. When externally supplied gas bubbles pass through the space between the HV electrode and ground electrode, a plasma spark discharge is more easily generated due to the lower conductivity of the gas in the bubble. In this case, however, the generation of the plasma spark discharge depends to some extent on the fluid dynamic motion of the gas bubbles. This motion is difficult to control since control must be exerted over each of the size, location and frequency of the bubbles. Thus, a better system than those employing gas bubbles from an external source is required to generate a highly reliable, pulsed plasma spark discharge at a predetermined location, time and frequency (for example, at 10 pulses per second).
The previous methods of using gas bubbles in a liquid to generate a pulsed plasma spark discharge utilized two electrodes, a HV electrode 102 and a ground electrode 104, as shown in FIG. 1, with their active surfaces facing each other. The gas bubbles 103 are provided by the gas injector 105 and pass between the HV electrode 102 and ground electrode 104. When a properly sized gas bubble 103 passes at a specific location between the HV electrode 102 and ground electrode 104, a pulsed plasma spark discharge is generated. However, if the size of a gas bubble 103 is too small, the pulsed plasma spark discharge may not be generated. Additionally, if the gas bubble 103 passes the HV electrode 102 and ground electrode 104 too quickly, the plasma spark discharge may not be generated. Since the size and velocity of a gas bubble 103 are not easily controlled in a water-carrying pipe, the HV electrode 102 and ground electrode 104 must be specially adapted to provide reliable plasma spark discharge. However, even the proposed geometry using a HV electrode 102 and ground electrode 104 facing each other with gas bubbles 103 passing between them does not provide an environment that can be precisely controlled in order to permit generation of a reliable plasma spark discharge.
Therefore, there is a need to control the introduction of gas bubbles to an apparatus for generating a plasma spark discharge more precisely. More specifically, it is desirable to provide more precise control of, for example, the size of the gas bubbles and the location of the gas bubbles relative to the HV electrode and ground electrode. More precise control would be able to provide a reliable plasma spark discharge in water such as is requried for consistent water treatment.
Additionally, there is a need for a more durable electrode in the application of plasma spark discharge to water and produced water. Specifically, some electrodes made of a sharp needle shape currently suffer from the problem of electrode erosion when employed in a plasma spark discharge reactor of the type shown in FIG. 1.
A durable electrode that is free of erosion, together with a high degree of control of the gas introduced to the system to facilitate generation of a plasma spark discharge is needed.
Such an improved plasma spark discharge reactor will permit generation of reliable pulsed plasma spark discharges at a more precise location in even a highly conductive liquid medium at a predetermined time and/or frequency (i.e., the number of pulses per second), as desired for effective water treatment.